Positron emission tomography (PET) is a diagnostic imaging modality that is used to non-invasively measure the bio-distribution of a radioactive tracer. In positron emission tomography, a positron emitting bare radioactive isotope or an isotope that has been attached to a biomolecule is injected into a patient or animal. A positron is emitted by the radioactive isotope and annihilates with an electron producing two photons in opposite directions. Each of the photons has approximately 511 keV of energy, corresponding to the rest mass of the positron and electron. These two annihilation photons escape the patient and interact in a scanner that is positioned around the patient.
A scanner is made of arrays of high-energy photon detectors that convert interactions in the detector into electrical signals that are processed on subsequent electronics driven by a computer. An example of a high-energy photon detector is a scintillation crystal that is connected to an optical photodetector such as a photomultiplier tube or solid state photomultiplier. The photon is classified as high-energy because the photon has an energy of 511 keV, or kilo electron volt, which is much larger than optical photons that have energies in the 2-5 eV range. The annihilation photon can interact in the high atomic number, dense scintillation crystal, which in turn emits optical photons that bounce inside of the scintillation crystal. The optical photons propagate inside the crystal and are absorbed by a photodetector converting the light into an electrical signal. The electrical signal is then processed by analog and digital electronic circuits and is recorded as an event. The data acquisition electronics process the signal and record the time, location of the crystal or crystals that absorbed the high-energy photon and any secondary interaction processes, and the energy of the incoming high-energy photon to storage. In positron emission tomography, the two photons are paired by their timestamps to produce a line-of-response (LOR) of the interaction. These LORs are processed by image reconstruction algorithms to produce 3-D images of the distribution of the radiotracer.
A time-of-flight scanner is one where the arrival times of the photons are recorded to such an extent that the annihilation location can be estimated with a given resolution. Because photons travel at the speed of light, the annihilation location can be estimated by the following equation: Δx=Δt/(2*c), where Δx is the location of the annihilation measured from the center of the line, Δt is the difference in time measured by the detectors, and c is the speed of light.